This invention relates to a nitride semiconductor element with a supporting substrate used for a light-emitting device such as a light emitting diode (LED), a laser diode (LD), etc., a photoreceptor such as a solar cell, a photo sensor, etc., an electronic device such as a transistor, a power device, etc., and a method for producing thereof. An attaching structure is employed as one of the methods for producing.
A nitride semiconductor is one of desirable candidate direct-band-gap semiconductor materials, however, it is difficult to produce a bulk of its single crystal. Therefore, hetero-epitaxial technology is usually employed to grow GaN on a different material substrate such as sapphire, SiC, etc. by metal-organic chemical vapor deposition (MOCVD) for the present. It was shown that sapphire is a preferable substrate for growing a high efficient light-emitting device of nitride semiconductor because of its stability at high temperature under atmosphere with ammonia in an epitaxial vapor deposition process compared with the other different material substrates. When a sapphire substrate is employed, a process for forming AIGaN layer as a buffer layer on the sapphire substrate at low-temperature around 600xc2x0 C. is usually employed to grow nitride semiconductor layers thereon. It can improve crystallinity of the nitride semiconductor layers.
Concretely Specifically, a nitride semiconductor element grown on a sapphire substrate is used for a blue LED, a pure-green LED with higher luminance than conventional LEDs, and an LD(laser diode). They are applied for a full-color display; traffic lights; an image scanner; light sources such as a light source for an optical disc, which is media, for example DVD, capable of memorizing a large-capacity of information; a light source for communication; a printer; etc. Further, it is anticipated to apply to an electronic device such as a field-effect transistor (FET).
Related Reference 1
Japanese Patent Laid-Open Publication Toku-Kai No. HEI 9-129932 (1997).
However, sapphire is a low thermal conductivity insulating material. Thus, the structure of nitride semiconductor element is limited. For example, in the case of conductive substrate such as GaAs or GaP, one of electric contact portions (terminals) can be disposed on the top surface of the semiconductor device, another contact portion can be disposed on the bottom. But, both of the electric contact portions of the light-emitting element grown on the sapphire substrate should be disposed on the top surface (the same plane side). Therefore, when an insulating material such as sapphire, etc. is employed as a substrate, it reduce the effective area of light-emission compared with a conductive substrate having the same area of substrate. In addition, when an insulating substrate is employed, it reduces the number of elements (chips) obtained from the same diameter of a wafer.
Further, a nitride semiconductor element with an insulating substrate such as sapphire is used as face-up type or face-down type. These types have both terminals in the same plane side, so that it increases current density locally. Then, it generates heat in the element (chip), so that it accelerates deterioration of the element. In addition, wires are required for both of pn terminals in a wire-bonding process for the terminals, so that it increases chip size. Therefore it reduces yield of chips. Additionally, sapphire has high hardness and a crystal structure with hexagonal system. So that when sapphire is employed as a substrate for growth, it is requires to break into chips by scribing the sapphire substrate. Thus, it requires an additional process compared with the other substrates.
Furthermore, recently, it has been available that an LED capable of emitting in ultra-violet region is in practical use. Generally, ultra-violet region is defined as wavelength of light-emission not more than 400 nm. The band gap of GaN is 365 nm. To shorten the wavelength not more than 365 nm, absorption of GaN of a contact layer, etc. may reduce the outgoing efficiency of the light extremely.
The present invention is devised to solve the above problems, and therefore, is aimed to at providing a high efficient nitride semiconductor element having an opposed terminal structure, whose terminals facing each other, without increasing its voltage, and a method for producing thereof. Further, it is another object to provide a high light-emitting power nitride semiconductor element even in ultra-violet region.
The nitride semiconductor element of the invention includes, at least a conductive layer, a first terminal, a nitride semiconductor with a light-emitting layer, and a second terminal, from a supporting substrate successively, wherein, the first terminal and a first insulating protect layer are interposed between the conductive layer and a first conductive type nitride semiconductor layer. The nitride semiconductor may include the first conductive type nitride semiconductor layer, the light-emitting layer, and a second conductive type nitride semiconductor layer, which has an asperity portion as a top layer thereof. When the supporting substrate is conductive material, it can provide the nitride semiconductor element with an opposed terminal structure. In addition, when the first terminal is a p-type terminal, it can improve the outgoing efficiency of the light. That is, the second conductive type nitride semiconductor element formed in the second terminal (n-type terminal) side, which is topside of the nitride semiconductor layer, is an n-type nitride semiconductor layer. In other word, the n-type nitride semiconductor layer side is the outgoing surface of the light. An n-type layer in the nitride semiconductor (especially GaN system semiconductor) is of low resistance, so that the size of the n-type terminal, the second terminal, can be downsized. Because downsizing the size of the n-type terminal reduce the area cutting off the light, it can improve the outgoing efficiency of the light. Additionally, the conventional nitride semiconductor element has a structure having both terminals in the same plane side, so that it is required to provide a p-pad terminal for the p-type terminal. When conductive material is employed as the supporting substrate in the invention, die-bonding to a package such as a lead frame with a conductive material can achieve continuity. Therefore the p-pad terminal can be eliminated, it can increase the area of light-emission. In addition, providing the first insulating protect layer can prevent short circuit, etc., so that it can improve yield and reliability. It can also simplify its producing process.
In the nitride semiconductor element of the invention, the first terminal and the first insulating protect layer are in contact with the first conductive type nitride semiconductor layer. The first terminal may be formed on the whole of the first conductive type nitride semiconductor layer, however, it should be appreciated that forming the first terminal partially and covering an opening portion with the first insulating protect layer can adjust the contact area between the first terminal and the first conductive type nitride semiconductor layer. In addition, forming the first terminal in a pattern such as a rectangular shape, lines, a square shape, a grid pattern, dots, a rhombus, a parallelogram, a mesh shape, a striped shape, a ramose shape branching from one into a plurality of branches, etc. can improve the outgoing efficiency of the light. When the first conductive type nitride semiconductor layer can have ohmic contact with the first terminal, either p-type terminal or n-type terminal can be employed as the first conductive type nitride semiconductor layer. The first conductive type nitride semiconductor layer is not restricted either in a single-layer or a multi-layer.
The first terminal includes at least one element selected from the group of. Ag, Rh, Ni, Au, Pd, Ir, Ti, Pt, W, and Al. Concretely, reflectivity of Ag, Al, Rh, Pd, and Au are 89%, 84%, 55%, 50%, and 24%, respectively. Thus, according to the reflectivity Ag is the most preferable material, however, it is preferable to employ Rh in view of ohmic contact when the first conductive type nitride semiconductor layer is p-type. Using the material can achieve low resistance, and can improve the outgoing efficiency of the light. The conductive layer is formed of eutectic, which includes at least one element selected from the group of Au, Sn, and In. Employing the eutectic material as the conductive layer can form the layers even at low temperature. The eutectic junction can attach at low temperature, so that it can achieve an effect for reducing warpage. Additionally, employing the structure of (intimate-contact layer)/(barrier layer)/(eutectic layer) formed of Au, Sn, Pd, In, Ti, Ni, W, Mo, Auxe2x80x94Sn, Snxe2x80x94Pd, Inxe2x80x94Pd, Tixe2x80x94Ptxe2x80x94Au, and Tixe2x80x94Ptxe2x80x94Sn, etc. from the first terminal side can prevent deterioration cause of the diffusion from the first terminal (p-type terminal, for example).
In the nitride semiconductor element of the invention, the first terminal and the second terminal are formed in an opposed terminal structure, and the second terminal is disposed on the portion corresponding to the rest of the portion, on which the first terminal is disposed. That is, in a view from the terminal-forming surface, both terminals do not overlap each other. Because both terminals do not overlap each other in a view from the terminal-forming surface, the emitted light can outgo effectively without being cut off by the second terminal (n-type terminal, for example). Thus, it can reduce the absorption of the emitted light by the second terminal. When the conductive type nitride semiconductor layer is n-type, it is preferable that the second terminal includes Al, such as Tixe2x80x94Al, Wxe2x80x94Al, for example. In the present invention, the opposed terminal structure is meant a structure, in which the first terminal and the second terminal are formed so as to face each other with interposing the nitride semiconductor.
In the nitride semiconductor element of the invention, the nitride semiconductor includes a second conductive type nitride semiconductor layer with an asperity portion as a top layer thereof. The asperity-forming (dimple processing) portion is provided in the outgoing side of the light. Forming the asperity on the surface can let the light, which does not outgo cause of the total internal reflection, outgo by varying the entry angle of the light at the asperity surface. It is anticipated that forming the asperity potion improve more than or equal to 1.5 times of the power compared with that without asperity. Its plane shape can be formed in a circle shape, polygonal shape such as a hexagonal shape or a triangle shape. In addition, the asperity also can be formed in a striped shape, a grid pattern, and a rectangular shape. It is preferable to form in a micro pattern for improving the outgoing efficiency of the light. In addition, it is preferable that its cross-sectional shape is a wave shape rather than a flat plane. Because it can improve the outgoing efficiency of the light compared with the square-cornered asperity. Additionally, it is preferable that the depth of the asperity is 0.2-3 xcexcm. It is more preferable that it is 1.0-1.5 xcexcm. It causes that it is less effective to improve the outgoing efficiency of the light, if the depth of the asperity is shallower than 0.2 xcexcm. If the depth is deeper than the above range, the resistance in the transverse direction may be increased. In addition, drawing out to form the asperity shape in a circle shape or a polygonal shape can improve its power with maintaining low resistance.
In the nitride semiconductor element of the invention, the nitride semiconductor layers except the light-emitting layer in the nitride semiconductor have a band gap larger than the light-emission band gap. It is more preferable that the nitride semiconductor layers except the light-emitting layer in the nitride semiconductor have a band gap more than or equal to 0.1 eV larger than the light-emission band gap. Thus, the emitted light can outgo without absorption.
In the nitride semiconductor element of the invention, the linear thermal expansion coefficient of the supporting substrate is 4-10xc3x9710xe2x88x926/K. Setting the coefficient of linear thermal expansion of the supporting substrate in the above range can prevent warpage or crack of the nitride semiconductor element. Because over the above range increase the warpage and the ratio of occurrence of the crack of the nitride semiconductor element or the supporting substrate sharply, it is required to set the difference of the thermal expansion coefficient of GaN within not more than 4-10xc3x9710xe2x88x926/K.
In the nitride semiconductor element of the invention, the supporting substrate includes at least one element selected from the group of Cu, Mo, and W. The characteristics of the supporting substrate are required to have conductivity, and the thermal expansion coefficient approximate to the nitride semiconductor element. The supporting substrate including the above metal satisfies these characteristics. In addition, it can improve the characteristics of LED or LD such as high thermal dissipation, and ease of chip separation.
In the nitride semiconductor element of the invention, the content of Cu in the supporting substrate is not more than 50%. While increasing the content of Cu improves thermal conductivity, increases thermal expansion coefficient. Therefore, it is more preferable that the content of Cu is not more than 30%. It is preferable to decrease thermal expansion coefficient for alloying with Cu. When Mo is alloyed with Cu contained therein, the content of Mo is more than or equal to 50%. Mo is low cost. In addition, when W is alloyed with Cu contained therein, the content of W is more than or equal to 70%. W can be diced easily. Employing such supporting substrate can make its thermal expansion coefficient closer to the nitride semiconductor, so that it can provide preferable characteristics for thermal conductivity. The supporting substrate exhibits conductivity, so that it is possible to apply a large amount of current.
The first insulating protect layer includes a metal layer, which includes at least one element selected from the group of Al, Ag, and Rh, is formed on the side of the first insulating protect layer not in contact with the nitride semiconductor. That is, the metal layer is interposed between the conductive layer and the first insulating protect layer (FIG. 4). Forming the metal layer at this position can improve the outgoing efficiency of the light. Because it can reflect the light, which mostly runs in the transverse direction in the LED, toward light-outgoing face side. The metal layer is in contact with the conductive layer.
The semiconductor light-emitting element includes the first terminal 3, the laminated semiconductor layer 2 with the light-emitting layer, and the second terminal 6 on or above the supporting substrate 11 successively. Here, the first terminal 3 is provided in the junction plane side with the supporting substrate 11 supporting the semiconductor layer 2. In addition, the second terminal 6 is provided the light-outgoing surface side of the semiconductor 2. In such light-emitting element, the light emitted from the light-emitting layer is not radiated only upward, or toward outgoing surface, but also in all direction. So that the light radiated downward in the light emitted from the light-emitting layer is absorbed by the other formed layers. On the other hand, the thickness of the semiconductor layer 2 formed in the semiconductor element is about several xcexcm to 10 xcexcm, while the length of the traverse direction is not less than 200 xcexcm, further more than 1 mm in wider one. Since the light transmitted longer distance until reflected at the side surface of the semiconductor, and so on, in the traverse direction than in the vertical direction, it is absorbed by the materials composing the semiconductor. Thus, the outgoing-efficiency of the light is reduced.
The semiconductor light-emitting element of the invention has: at least the conductive layer 13; the first terminal 3; the semiconductor 2, which includes the first conductive type semiconductor layer 2a in the contact boundary side with the first terminal, the light-emitting layer thereon, and the second conductive type semiconductor layer 2c further thereon in the light-outgoing surface side; and the second terminal on or above the supporting substrate 11 successively. The semiconductor light-emitting element further has the first protect layer 4, which has a contact boundary region with the semiconductor 2 and/or a region extending from the contact boundary in traverse direction of the semiconductor 2.
It is meant also to include even interposing an interposition layer between the first protect layer 4 and the semiconductor 2 that the first protect layer 4 has the contact boundary region with the semiconductor 2, as long as the first protect layer 4 and the laminated semiconductor layer 2 has optical connection transmittable of the light. Additionally, in the first protect layer 4, the region extending from the contact boundary in traverse direction of the semiconductor 2 is shown the region, in which the protect layer 4 is not in contact with the semiconductor 2, extended to the outside of the semiconductor layer 2 (FIG. 4, etc.). The first protect layer 4 is only to required to have the effect as a light-transmitting layer transmittable of the light emitted from the light-emitting layer. In addition, it works as insulating layer with the effect for preventing a leak current and for current convergence (current blocking). The light is transmitted from the light-emitting layer 2b downward, and moves into the first protect layer 4. The transmitted light is reflected upward at the boundary with a layer having reflection effect, and outgoes as an outgoing light through the extending region, which is provided outside of the semiconductor layer 2 as a light-outgoing surface. The light transmitted from the light-emitting layer 2b of the semiconductor 2 moves into the first protect layer 4, and it is repeatedly reflected at the side surfaces and the bottom surface of the first protect layer 4, then most of the light outgoes as the outgoing light upwardly though the top surface of the extending region. The thickness of the first protect layer is less than the thickness of the semiconductor in growth direction. Thus, the absorption and loss in the light-emitting element can be reduced, and the outgoing efficiency of the light from the light-emitting element is improved. It is preferable to select a material with low absorption coefficient as the first protect layer 4. The extending region, which is a light-outgoing path transmitting the light moving into the first protect layer 4 connected optically with the semiconductor layer 2, has the effect of guiding the emitted light outward before the light reflected repeatedly inside of the semiconductor laminated body is absorbed caused of the internal absorption. The sub light, which outgoes from the extending region corresponding to outside of the semiconductor light-emitting element, is added to the main light, which outgoes from the upper part of the first terminal 3, so that the external quantum efficiency can be improved. Concretely, the conductive layer 13 works as the layer with reflection effect, however, it is preferable to interpose a reflecting layer between the first protect layer 4 and the conductive layer 13. It is possible to reduce the loss at the reflection in the first protect layer 4.
Forming an asperity surface on the top surface of the extending region in the first protect layer 4 by etching and so on can improve the outgoing efficiency of the light from the surface. As another constitution, forming a protect layer 40 with refractive index n3 on the top surface of the extending region can also achieve the same result. When the refractive index difference between the formed protect layer 40 and the refractive index n2 of the first protect layer 4 is less than the refractive index difference between the refractive index n1 of the semiconductor layer 2 and the refractive index n2 of the first protect layer 4, a large part of the light outgoes toward less refractive-index-difference side. Therefore, a large part of the light moves into the first protect layer 4 having the surface exposed outside, and it is possible to improve the outgoing efficiency of the light.
Further, in the semiconductor light-emitting element, at least one first terminal 3 and the first protect layer 4 is formed by turns on the surface of the semiconductor in the supporting substrate side. It is preferable that the semiconductor light-emitting element has a reflecting layer under the first protect layer 4 (FIG. 12F). The light from the light-emitting layer is reflected at the boundary a between the first conductive type semiconductor layer 2a and the first terminal 3. In addition, the light-emitting layer passes through the boundary between the first conductive type semiconductor layer 2a and the first protect layer 4, and the light from the light-emitting layer is reflected at the boundary b between the first protect layer 4 and the conductive layer 13. The first terminal 3 absorbs the light. To reduce this absorption of the light, reducing the reflectivity at the boundary between the semiconductor 2 and the first protect layer 4, and increasing the reflectivity at the boundary b guides the light into the first protect layer 4, thereby the reflecting layer or the conductive layer 13 formed under the first protect layer reflects the light. Thus, it is possible to improve the outgoing efficiency of the light. It is preferable that the reflectivity of the first protect layer 4 is lower than the first terminal 3, and is formed of a material with high transmittance of the light.
Both of the boundary a between the first conductive type semiconductor layer 2a and the first terminal 3, and the boundary b between the first protect layer 4 and the conductive layer 13 are formed as an asperity portions. Here, the boundaries a, b are the surfaces with the effect as the light-reflecting surface reflecting the light from the light-emitting layer 2b. The first protect layer 4 is a transparent layer. However, the first terminal 3 in contact with the side surface of the first protect layer 4 and the boundary b with the conductive layer 13 in contact with the back surface of the first protect layer 4 can reflect the light. Recess portions as the boundaries b and projecting portions as the boundaries a are provided in traverse direction (FIG. 12D). It is appreciated that the reflecting layer shown in FIG. 12D, etc, may be omitted.
Providing the asperity portion can improve the outgoing efficiency of the light from the semiconductor to the outside. The reasons is that the light, which is transmitted downward originally, is reflected or scattered with increasing the vertical component of the transmittance. That is, the light is scattered at the asperity portion so as to run upward before it is transmitted for long distance in the traverse direction. Most of the light with the high traverse component of the transmittance is absorbed in the semiconductor, However asperity portion scatters the light from the light-emitting layer in all directions divergently, then can change the light with vertical component of the transmittance. Optical connection between such asperity portion and the extending region of the first protect layer 4 as mentioned above further can improve the outgoing efficiency of the light.
The first terminal 3 and the first protect layer 4 are provided under the same surface of the first conductive type semiconductor layer 2a. Here, while the first conductive type semiconductor layer 2a may have the bumps and dips of the asperity formed by xe2x80x9cas-grownxe2x80x9d or suitable micro process on the first-terminal-forming surface, it is preferable that the surface is flat. If the asperity portion is formed on the semiconductor by etching, the semiconductor has not some little damage. Accordingly, the life characteristics shall be reduced. In the invention, the asperity portion is not formed by etching, but also formed by combining materials. Therefore, the outgoing efficiency of the light can be improved without etching damage or reduction of the life characteristics.
The first protect layer 4 has a multi-layer structure composed of at least two layers. The boundary surface between the layers is formed in asperity surface. It is preferable that the asperity surface is inclined. The first protect layer 4 has the area in the semiconductor larger than the first terminal 3 in the traverse direction of the semiconductor 2. Accordingly, the light transmitted in the first protect layer is high ratio of the whole emitted light. It is very important to change the light, which moves into the first protect layer 4 once, upward, thereby the light outgoes. To achieve it, forming the first protect layer 4 in the multi-layer structure composed of at least two layers, and forming the asperity in the first protect layer 4 scatter the light, which moves into the first protect layer 4, at the boundary to change its direction upwardly. The first protect layer is composed of materials such as SiO2, Al2O3, ZrO2, TiO2, Nb2O5. For example, the first protect layer 4 is formed in a two-layer structure composed of Nb2O5 in the boundary side 4b, and SiO2 as a lower the layer 4a. The asperity portion is provided between the two layers to effect diffusion in the protect layer (FIG. 12F).
The nitride semiconductor element of the invention has the first terminal 3 and the second terminal 6 of the opposed terminal structure (FIG. 3, FIG. 12, etc.). As mentioned above, it is preferable that the second terminal is disposed on the portion corresponding to rest of the position, on which the first terminal is disposed, however, it is not specifically limited, for example, the second terminal may be disposed on the portion corresponding to the first terminal portion partially. In FIG. 3D, when the second terminal 6 is an n-type terminal, the current flows in wide area of the nitride semiconductor in the second terminal side, or n-type nitride semiconductor 2c. On the other hand, the current flows in narrow area of the nitride semiconductor in the first terminal 3 side, or p-type nitride semiconductor 2a, so that the first terminal is widely formed in the surface of the nitride semiconductor. To achieve efficient outgoing of the light, it is preferable that the second terminal is formed in a shape surrounding the top surface of the light-outgoing portion of the semiconductor 2. However, the terminal-forming area of the second terminal 6 can be small, both terminals may partially overlap each other as long as no cutting off a large amount of the light (FIG. 12E).
In addition, in the invention, the bumps and the dips of the asperity portion formed in the light-outgoing surface are formed in square shapes or rectangular shapes with square corners, mesa shapes or reverse-mesa shapes with inclined surfaces, or the like. It is preferable that the shape of the asperity portion has inclined surfaces.
The semiconductor 2 is nitride semiconductor in the invention. The nitride semiconductor is a semiconductor compound including nitrogen, The nitride semiconductor is direct-band-gap semiconductor. It has efficiency of light-emission much higher than indirect-band-gap semiconductor. Additionally, when it is formed of a semiconductor compound including group III element such as In, Ga, Al, the semiconductor light-emitting element capable of light-emission in the short wavelength region (300-550 nm) including ultra-violet region can be provided.
The light-emitting layer has a quantum well structure, which includes at least a well layer of AlaInbGa1xe2x88x92axe2x88x92bN (0=a=1, 0=b=1, a+b=1) and a barrier layer if AloIndGa1xe2x88x92cxe2x88x92dN (0=c=1, 0=d=1, c+d=1). The quantum well structure can provide the light-emitting element with high light-emission efficiency. The quantum well structure can be either a single quantum well structure or multi-quantum-well structure. In addition, it is preferable for achieving high power that b of the in composition of the well layer is set as 0 less than b=0.3. Because the mixture ratio b of In is higher, the crystallinity is prone to be uneven in the plane cause of segregation of the crystal, and preferable portions are interspersed in the plane. Additionally, it is prone to makes less linearity of the current-output characteristics and to become saturated easily. However, setting within the above range of the in composition can apply a large amount of current, so that it provide the advantage in the invention.
The light-emitting layer has a quantum well structure, which includes at least a well layer of AlaInbGa1xe2x88x92axe2x88x92bN (0 less than a=1, 0 less than b=1, a+b less than 1) and a barrier layer of AlcIndGa1xe2x88x92cxe2x88x92dN (0 less than c=1, 0 less than d=1, c+d less than 1), and the first conductive type semiconductor layer is disposed in one side of the principal plane of the light-emitting layer, the second conductive type semiconductor layer, which includes Al, is disposed in another side of the principal plane of the light-emitting layer. In addition, in the range not more than 420 nm (near-ultra-violet region), which is low luminosity, b of the in composition is set as around 0 less than b=0.1. In the range not more than 380 nm (ultra-violet region), a of the Al composition is set as around 0.01=b=0.2.
The second conductive type nitride semiconductor layer includes at least two layers, one layer of said two layers, which is disposed in the second terminal side, is formed of AleGa1xe2x88x92eN, and another layer of said two layers, which is disposed in the light-emitting layer side, is formed of AlfGa1xe2x88x92fN, wherein, the impurity concentration of the AleGa1xe2x88x92eN layer is higher than the AlfGa1xe2x88x92fN layer.
In the invention, AlaInbGa1xe2x88x92axe2x88x92bN (0=a=1 0=b=1, a+b=1) can be employed as an active layer of the nitride semiconductor element. It can be applied to elements emitting light with various wavelengths, in the light-emitting element of InAlGaN system. Especially, the nitride semiconductor element in the ultra-violet region, which is not more than 380 nm, further has a particular advantageous. The invention provide a method for producing the nitride semiconductor element, which has a active layer having a quantum well structure with a well layer formed of a quaternary mixed crystal of InAlGaN and a barrier layer formed of nitride semiconductor including at least Al, capable of use even for short wavelength range, which is not more than 380 nm, appropriately. Because the well layer of the above active layer is formed of a quaternary mixed crystal of InAlGaN, it can minimize the number of the composition elements, and can reduce deterioration of the crystallinity, and further can improve light-emission efficiency. In addition, the band gap of the barrier layer formed of the nitride semiconductor including at least Al can be wider than the well layer, so that the active layer with the quantum well structure suitable for the wavelength of the light-emission can be formed, and can be maintain preferable crystallinity in the active layer.
Further, especially in the nitride semiconductor element for the ultra-violet region, which is not more than 380 nm, it is required to grow a GaN layer on or above a substrate, a buffer layer at high temperature for obtaining the nitride semiconductor element with preferable crystallinity. If a light-emitting layer (active layer) is grown without growing this layer, its crystallinity may be very poor. Therefore, in such nitride semiconductor light-emitting element, its light-emitting power is quit low, so that it is not appropriate for practical use, Thus, forming a high-temperature-grown layer of GaN can provide the nitride semiconductor element with preferable crystallinity. However, when a GaN layer is included as a primary layer or the high-temperature-grown layer, this GaN layer absorbs a part of the light from the active layer cause of the self-absorption of the GaN in the ultra-violet region. In the invention, the substrate for growing, the buffer layer, and high-temperature-grown layer of GaN are eliminated after attaching the conductive substrate, so that it can maintain the crystallinity of the nitride semiconductor element appropriately, and can reduce self-absorption.
It should be appreciated that a composition-graded layer may further be formed on or above the high-temperature-grown layer. The composition-graded layer is useful for the LED, which does not have GaN playing a role of recovering crystallinity, capable of emitting in ultra-violet region. It can laminate the nitride semiconductor layer with less defect and high crystallinity. In addition, it should be appreciated that the composition-graded layer may be formed with modulated-doping so as to grade impurity concentration affecting its conductivity. When the nitride semiconductor layer of Si-doped AlGaN is formed thereon for example, the composition-graded layer is formed in the structure graded from undope to the impurity concentration similar to the Si-concentration of the n-type cladding layer. It can laminate the nitride semiconductor layer with further less defect and high crystallinity. Additionally, it should be appreciated that the composition-graded layer may be formed with graded from a high-impurity-concentration region to an undoped layer.
In the nitride semiconductor element of the invention, especially in the light-emitting element, it should be appreciated that a coating layer or a molding material including a fluorescent material, which can absorb a part of or the whole of the light from the active layer then can emit light with different wavelength, may be formed on the nitride semiconductor element with attached to supporting substrate. It can emit light with various wavelengths. Examples of the fluorescent material are shown as follows. SrAl2O4:Eu; Y2SiO5:Ce, Tb; MgAl11O19:Ce, Tb; Sr7Al12O25:Eu; and (at lease one element selected from the group of Mg, Ca, Sr, and Ba)Ga2S4:Eu are can be employed as a greenish fluorescent material, in addition, Sr5(PO4)3Cl:Eu; (SrCaBa)5(PO4)3Cl:Eu; (BaCa)5(PO4)3Cl:Eu; (at lease one element selected from the group of Mg, Ca, Sr, and Ba)2B5O9Cl:Eu,Mn; and (at lease one element selected from the group of Mg, Ca, Sr, and Ba)(PO4)6Cl2:Eu,Mn can be employed as a bluish fluorescent material. Additionally, Y2O2S:Eu; and La2O2S:Eu; Y2O3:Eu; Gd2O2S:Eu are can be employed as a reddish fluorescent material. Especially, including YAG can emit white light, so that it can be applied to a light source for illumination, etc. widely. The YAG is represented in (Y1xe2x88x92xGax)3(Al1xe2x88x92yGay)5O12:R (R is at lease one element selected from the group of Ce, Tb, Pr, Sm, Eu, Dy, and Ho. 0 less than R less than 0.5.), for example (Y0.8Gd0.2)3Al5O12:Ce or Y3(Al0.8Ga0.2)5O12:Ce or the like. In addition, with regard to the fluorescent material, which can absorb a part of or the whole of the light then can emit light with different wavelength; the material, which can absorb a part of or the whole of visible light then can emit light with different wavelength, is limited. Therefore, there is a problem of material selectivity. However, many materials, which can absorb a part of or the whole of the ultra-violet light then can emit light with different wavelength, are known, so that it can select the material according to various applications. One reason to be able to select the material is high efficiency of light-conversion of the fluorescent material absorbing ultra-violet light compared with the efficiency of light-conversion of visible light. White light widely provides possibility such as obtaining white light with high color rendering or the like. The invention can provide a nitride semiconductor light-emitting element with less self-absorption. Further, the invention can provide a white light-emitting element with extremely high efficiency of conversion by coating the fluorescent material.
Fluorescent Material
It is preferable that the fluorescent material used in the invention has particle size with center particle size in the range of 6-50 xcexcm. It is more preferable that the center particle size is in the range of 15-30 xcexcm. The fluorescent material with such particle size has a high absorption coefficient, high converting efficiency, and wide range of excited light wavelength. Since the fluorescent material with the particle size less than 6 xcexcm relatively tends to form aggregate, they sediment in the liquid resin cause of their density, so that it might reduces transmittance of the light, further, its absorption coefficient and the converting efficiency might be poor, and its range of excited light wavelength might be narrow.
In the invention, the particle size is meant a value obtained by the volume-base particle size distribution curve. The volume-base particle size distribution curve is measured by the laser diffraction and scattering method. Specifically, it can be obtained with measurement of sodium hexametaphosphate aqueous solution, in which each substance is dispersed, with 0.05% concentration by laser diffraction type particle size distribution analyzer (SALD-2000A), in the measurement particle size range 0.03 xcexcm-700 xcexcm, under circumstance temperature 25xc2x0 C. and humidity 70%. In the invention, the center particle size is meant a particle size value when the integrated value reaches 50% in the volume-base particle size distribution curve. It is preferable that the content of the fluorescent material with this center particle size is high. It is preferable its content is 20-50%. Employing the fluorescent material with less variation of the particle size can reduce variation of the color, so that the light-emitting device with preferable contrast can be provided.
Yttrium-Aluminum-Oxide System Fluorescent Material
The light-emitting device of the invention employs a fluorescent material based on the yttrium-aluminum-oxide based fluorescent material (YAG system fluorescent material) activated with cerium (Ce) or praseodymium (Pr), which can light with being exited by the light emitted from the semiconductor light-emitting element with the light-emitting layer of the nitride semiconductor.
As the concrete yttrium-aluminum-oxide based fluorescent material, YAlO3:Ce, Y3Al5O12:Ce(YAG:Ce), Y4Al2O9:Ce, or mixture of them may be usable. The yttrium-aluminum-oxide based fluorescent material may include at least one element selected from the group consisting of Ba, Sr, Mg, Ca, and Zn. In addition, adding Si can control or reduce reaction of the crystal growth to make particles of the fluorescent material uniform.
In this specification, the yttrium-aluminum-oxide based fluorescent material activated with cerium is meant in a broad sense, and it includes a fluorescent material capable of fluorescent action having at least one element selected from the group consisting of Lu, Sc, La, Gd, and Sm, with which substitute all or part of the yttrium and/or at least one element selected from the group consisting of Ba, Ti, Ga, and In, with which substitute all or part of the aluminum.
More specifically, it may be a photo-luminescent fluorescent material having a general formula (YzGd1xe2x88x92z)3Al5O12:Ce (where 0 less than z less than =1), or a photo-luminescent fluorescent material having a general formula (Re1xe2x88x92aSma)3Rexe2x80x25O12:Ce (where 0 less than =a less than 1; 0 less than =b less than 1; Re is at least one element selected from the group consisting of Y, Gd, La, and Sc; and Rexe2x80x2 is at least one element selected from the group consisting of Al, Ga, and In.). Since the fluorescent material has a garnet stricture, it is impervious to heat, light, and moisture. Its peak of excitation spectrum can be around 450 nm. The peak of light-emission is around 580 nm, and the distribution of the light-emission spectrum is broad in which the foot of the distribution extends to about 700 nm.
Gd (gadolinium) may be added to the crystal lattice of the photo-luminescent fluorescent material to improve excited light-emission efficiency in the long-wavelength range over 460 nm. Increasing the Gd content shifts the peak wavelength of the light-emission toward long-wavelength side, and also overall wavelength of the light-emission toward long-wavelength side. In other words, if reddish light-emission color is needed, increasing the amount of substitutive Gd can achieve it. On the other hand, the more Gd is increasing, the less luminance of the photo-luminescent by the blue light. Other elements such as Tb, Cu, Ag, Au, Fe, Cr, Nd, Dy, Co, Ni, Ti, Eu may be added as well as Ce, if desirable. If Al in the composition of the yttrium-aluminum-garnet fluorescent material with the garnet structure is partially substituted with Ga, the wavelength of the light-emission shifts toward longer-wavelength region. In contrast, if Y in the composition is partially substituted with Gd shifts the wavelength of the light-emission toward longer-wavelength region.
If a part of Y is substituted with Gd, it is preferable that the percentage of substitutive Gd is less than 10%, and the composition ratio or substitution is 0.03-1.0. In case the percentage of substitutive Gd were less than 20%, the light in the green range would be high and the light in the red range be less. However, increasing content of the Ce can compensate the light in the red range, so as to be desired color tone without reduction of the luminance. Such composition can achieve preferable temperature characteristics, and improve reliability of the light-emitting diode. In addition, to use the photo-luminescent fluorescent material adjusted to emit the light in the red range, the light-emitting device can emit the intermediate color such as pink.
A material for the photo-luminescent fluorescent material can be obtained by mixing oxides or compounds sufficiently, which can become oxide at high temperature easily, as materials of Y, Gd, Al, and Ce according to stoichiometry ratio. The mixed material also can be obtained by mixing: coprecipitation oxides, which are formed by firing materials formed by coprecipitating solution dissolving rare-earth elements, Y, Gd, and Ce, in acid according to stoichiometry ratio with oxalic acid; and an aluminum oxide. After mixing the mixed material and an appropriate amount of fluoride such as barium fluoride, ammonium fluoride as flux, inserting them in to a crucible, then burning them at temperature 1350-1450xc2x0 C. in air for 2-5 hours, as a result, a burned material can be obtained. Next, the burned material is crushed in water by a ball mill. Then washing, separating, drying it, finally sifting it through a sieve, the photo-luminescent fluorescent material can be obtained.
In the light-emitting device of the invention, the photo-luminescent fluorescent material may be a substance mixed two or more kinds of the yttrium-aluminum-garnet fluorescent material activated with cerium, or can be a substance mixed the yttrium-aluminum-garnet fluorescent material activated with cerium and the other fluorescent materials. Mixing two kinds of the yttrium-aluminum-oxide system fluorescent materials, which have different amount of the substitution from Y to Gd, can achieve the desired color light easily. Especially, when the fluorescent material with higher content of the amount of the substitution is the above fluorescent material, and the fluorescent material with lower content of or without the amount of the substitution is the fluorescent material with middle particle size, both the color rendering characteristics and the luminance can be improved.
Nitride System Fluorescent Material
The fluorescent material used in the invention is a nitride system fluorescent material, which includes N, and can include at least one element selected from the group consisting of Be, Mg, Ca, Sr, Ba, and Zn, at least one element selected from the group consisting of C, Si, Ge, Sn, Ti, Zr, Hf, and is activated with at least one element selected from the group consisting rare-earth elements. In the invention, the nitride system fluorescent material is meant a fluorescent material, which is capable of absorbing the visible, ultra-violet light emitted from the light-emitting element, or the fluorescence from the YAG fluorescent material partially and of emitting a excited light. The fluorescent material according to the invention is silicon nitride such as Mn-added Srxe2x80x94Caxe2x80x94Sixe2x80x94N:Eu; Caxe2x80x94Sixe2x80x94N:Eu; Srxe2x80x94Sixe2x80x94N:Eu; Srxe2x80x94Caxe2x80x94Sixe2x80x94Oxe2x80x94N:Eu; Caxe2x80x94Sixe2x80x94Oxe2x80x94N:Eu; and Srxe2x80x94Sixe2x80x94Oxe2x80x94N:Eu systems. The basic component elements of the fluorescent material is represented in the general formulas LXSiYN(2/3X+4/3Y):Eu or LXSiYOZN(2/3X+4/3Yxe2x88x922/3Z):Eu (where L is any one element of Sr, Ca, Sr, or Ca). It is preferable that X and Y in the general formulas are X=2, Y=5, or X=1, Y=7, however, it is not specifically limited. As concrete basic component elements, it is preferable that fluorescent materials represented in Mn-added (SrXCa1xe2x88x92X)2Si5N8:Eu; Sr2Si5N8:Eu; Ca2Si5N8:Eu; SrXCa1xe2x88x92XSi7N10:Eu; SrSi7N10:Eu; and CaSi7N10:Eu are employed. Here, the fluorescent material may include at least one element selected from the group consisting of Mg, Sr, Ca, Ba, Zn, B, Al, Cu, Mn, Cr, and Ni. In addition, the invention is not limited in these materials.
L is any one element of Sr, Ca, Sr, or Ca. The composition ratio of Sr and Ca can be varied, if desirable.
Employing Si in composition of the fluorescent material can provide the low cost fluorescent material with preferable crystallinity.
Europlum, which is a rare-earth element, is employed as center of fluorescent. Europium mainly has a divalent or trivalent energy level. The fluorescent material of the invention employs Eu2+ as the activator against the base material of alkaline-earth-metal system silicon nitride. Eu2+ tends to be subject to oxidation. Trivalent Eu2O3 is available on the market. However, O in Eu2O3 available on the market is too active, it is difficult to obtain the preferable fluorescent material. It is preferable to use Eu2O3, from which O is removed out of the system. For example, it is preferable to use europium alone or europium nitride. In addition, when Mn is added, it is not always required.
Added Mn accelerates diffusion of Eu2+, and improves light-emitting efficiency such as light-emission luminance, energy efficiency, or quantum efficiency. Mn is included in the material, or is added in the process as Mn alone or Mn compounds, then is burned with the material. In addition, after burned, Mn does not remain in the basic component elements or remains much less than the original content even included. It is considered that Mn flies away in the burning process.
The fluorescent material includes at least one element selected from the group consisting of Mg, Sr, Ca, Ba, Zn, B, Al, Cu, Mn, Cr, and Ni in the basic component elements or with the basic component elements. These elements have the effect increasing the particle size, or improve light-emitting luminance. In addition, B, Al, Mg, Cr, and Ni have the effect reducing persistence.
Such nitride system fluorescent materials is capable of absorbing the blue light emitted from the light-emitting element partially and of emitting a excited light in the region yellow to red. Employing the nitride system fluorescent material with the YAG system fluorescent material in the above light-emitting device can provide the light-emitting device capable of emitting a warm white color by mixing the blue light emitted from the light-emitting element and the light in the region yellow to red from the nitride system fluorescent material. It is preferable that the other fluorescent materials except the nitride system fluorescent material include the yttrium-aluminum-oxide system fluorescent materials activated with cerium. Including the yttrium-aluminum-oxide system fluorescent materials can adjust desired chromaticity. The yttrium-aluminum-oxide system fluorescent material activated with cerium is capable of absorbing the blue light emitted from the light-emitting element partially and of emitting an excited light in the region yellow. The blue light emitted from the light-emitting element and the yellow light of the yttrium-aluminum-oxide system fluorescent material are mixed. Mixing the yttrium-aluminum-oxide system fluorescent material and the fluorescent material capable of emitting red light in the color converting layer, and combining them with blue light emitted from the light-emitting element can provide the light-emitting device emitting white light as mixed color light. It is preferable that its chromaticity of the white-light-emitting device is on blackbody radiation locus in the chromaticity diagram. The light-emitting device emitting whitish mixed light is aimed at improving a special color-rendering index of R9. In a conventional white-light-emitting device combining the bluish-light-emitting element and the yttrium-aluminum-oxide system fluorescent material activated with cerium, its special color-rendering index of R9 around color temperature Tcp=4600 K in nearly zero, and a red color component is not enough. Accordingly, it is required to improve special color-rendering index of R9. In the invention, employing the fluorescent material capable of emitting red light with the yttrium-aluminum-oxide system fluorescent material can improve special color-rendering index of R9 around color temperature Tcp=4600 K to about 40.
Next, a process for producing the fluorescent material ((SrXCa1xe2x88x92X)2Si5N8:Eu) used in the invention will be described as follows. However, the process for producing in the invention is not specifically limited. The above fluorescent material includes Mn, O.
1. The materials Sr and Ca are pulverized. It is preferable to use Sr and Ca alone as the materials. However, an imide compound, an amide compound, or the like also can be employed. In addition, the materials Sr, Ca may include B, Al, Cu, Mg, Mn, Al2O3, and so on. The materials Sr and Ca are pulverized in the glove box under atmosphere with argon. It is preferable that Sr and Ca have the average particle size about 0.1 xcexcm-15 xcexcm, however it is not specifically limited. It is preferable that the purity of Sr and Ca is more than or equal to 2N, however it is not specifically limited. To achieve preferable mixture, at least one element of metal Ca, metal Sr, and metal Eu is alloyed, and nitrided, then pulverized for using as the materials,
2. The material Si is pulverized. It is preferable to use Si alone as the materials. However, a nitride compound, an imide compound, an amide compound, or the like, for example Si3N4, Si(NH2)2, and Mg2Si, etc. also can be employed. It is preferable that the purity of the material Si is more than or equal to 3N, however the material may include compounds such as Al2O3, Mg, metal boride (CO3B, Ni3B, CrB), manganese oxide, H4BO3, B2O3 Cu2O, and CuO. Si is also pulverized in the glove box under atmosphere with argon or nitride, similar to the material Si and Ca. It is preferable that the Si compound has the average particle size about 0.1 xcexcm-15 xcexcm.
3. Subsequently, the materials Sr and Ca are nitrided under atmosphere with nitrogen. The equations, as Equation 1 and Equation 2, are
3Sr+N2xe2x86x92Sr3N2xe2x80x83xe2x80x83(Equation 1)
3Ca+N2xe2x86x92Ca3N2xe2x80x83xe2x80x83(Equation 2)
Sr and Ca are nitrided under atmosphere with nitrogen at 600-900xc2x0 C. for about 5 hours. Sr and Ca are nitrided with mixed together, or are nitrided individually. Finally, a strontium nitride and a calcium nitride are obtained. It is preferable that the strontium nitride and the calcium nitride have high purity. However, a strontium nitride and a calcium nitride on the market also can be employed.
4. The material Si is nitrided under atmosphere with nitrogen. The equation, as Equation 3, is
3Si+2N2xe2x86x92Si3N4xe2x80x83xe2x80x83(Equation 3)
Silicon Si is also nitrided under atmosphere with nitrogen at 600-900xc2x0 C. for about 5 hours. Finally, a silicon nitride is obtained. It is preferable that the silicon nitride used in the invention has high purity. However, a silicon nitride on the market also can be employed.
5. The strontium nitride and the calcium nitride, or the strontium-calcium nitride is pulverized. The strontium nitride, the calcium nitride, and the strontium-calcium nitride are pulverized in the glove box under atmosphere with argon or nitrogen.
The silicon nitride is pulverized similarly. In addition, the europium compound Eu2O3 is also pulverized similarly. Here, the europium oxide is employed as the europium compound, however metal europium, a europium nitride, or the like, can be employed. An imide compound, an amide compound, or the like can be employed as the material Z. It is preferable that the europium oxide has high purity. However, the europium oxide on the market also can be employed. It is preferable that the alkaline-earth-metal nitride, the silicon nitride, and the europium oxide have the average particle size about 0.1-15 xcexcm.
The above materials may include at least one element selected the group consisting of Mg, Sr, Ca, Ba, Zn, B, Al, Cu, Mn, Cr, O, and Ni. In addition, the above elements such as Mg, Zn, and B may be mixed with adjusting content in the processes below. These compounds can be added in the materials alone, normally they are added in the form of compounds. Such compounds are H3BO3, Cu2O3, MgCl2, MgO.CaO, Al2O3, metal boride (CrB, Mg3B2, AlB2, MnB), B2O3, Cu2O, CuO, and so on.
6. After pulverized, the strontium nitride, the calcium nitride, and the strontium-calcium nitride, the silicon nitride, and the europium compound Eu2O3 are mixed, and added with Mn. Since these mixtures undergo oxidation easily, they are mixed under atmosphere with argon or nitrogen in a glove box.
7. Finally, the mixtures of the strontium nitride, the calcium nitride, and the strontium-calcium nitride, the silicon nitride, and the europium compound Eu2O3 are burned under atmosphere with ammonia. Burning them can provide the fluorescent material represented in formula Mn-added (SrXCa1xe2x88x92X)2Si5N8:Eu. In addition, the ratio of each material can be changed so as to obtain composition of the desirable fluorescent material.
A tube furnace, a small furnace, a high-frequency furnace, a metal furnace, or the like can be used for burning. The burning is performed at burning temperature in the range 1200-1700xc2x0 C., however it is preferable that the burning temperature is at 1400-1700xc2x0 C. It is preferable to use one-stage burning, in which temperature rises slowly and burning is performed at 1200-1500xc2x0 C. for several hours. However, Two-stage burning (multi-stage burning), in which first-stage burning is performed at 800-1000xc2x0 C., and temperature rises slowly, then second-stage burning is performed at 1200-1500xc2x0 C., also can be used, it is preferable that the materials of the fluorescent material are burned in a crucible or a boat of a boron nitride (BN) material. Instead of the crucible of a boron nitride material, a crucible of alumina also can be used.
The desired fluorescent material can be obtained by the above method.
The nitride system fluorescent material is used as the fluorescent material capable of emitting reddish light in the light-emitting device as mentioned above. However, the light-emitting device can have the above YAG system fluorescent material and the fluorescent material capable of emitting reddish light. Such the fluorescent material capable of emitting reddish light is a fluorescent material, which can emit excited light by the light with wavelength 400-600 nm, for example Y2O2S:Eu, La2O2S:Eu, CaS:Eu, SrS:Eu, ZnS:Mn, ZnCdS:Ag,Al, ZnCdS:Cu, Al, and so on. Using the fluorescent material capable of emitting reddish light with the YAG system fluorescent material can improve color rendering o the light-emitting device.
Regarding the YAG system fluorescent material and the fluorescent material capable of emitting reddish light, for representative example the nitride system fluorescent material, formed as mentioned above, one layer of the color-converting layer in the side end surface of the light-emitting element includes two or more kinds of them, or two layers of the color-converting layer include one or more kinds of them respectively. Such constitution can provide mixed color light from different kinds of the fluorescent materials. In this case, it is preferable that each kind of the fluorescent materials has similar average particle size and similar shape for mixing the light from each kind of the fluorescent materials, and for reducing color variation. In addition, since the light converted its wavelength by the YAG system fluorescent material is partially absorbed by the nitride system fluorescent material, it is preferable that the nitride system fluorescent material is provided in the position closer to the side end surface of the light-emitting element than the YAG system fluorescent material. Accordingly the light converted its wavelength by the YAG system fluorescent material can avoid to be absorbed partially by the nitride system fluorescent material. Therefore, the color rendering of the mixed light of the YAG system fluorescent material and can be improved compared with mixing both fluorescent materials together.
The method of the invention for producing a nitride semiconductor element having at least a conductive layer, a first terminal, a nitride semiconductor with a light-emitting layer, and a second terminal, from a supporting substrate successively, comprising: a growing step for growing the nitride semiconductor having at least a second conductive type nitride semiconductor layer, the light-emitting layer, and a first conductive type nitride semiconductor layer, on a different material substrate; subsequently, a attaching step for attaching the supporting substrate to the first conductive type nitride semiconductor layer side of the nitride semiconductor with interposing the first terminal between them; and subsequently, a different-material-substrate-eliminating step for eliminating the different material substrate so as to expose the second conductive type nitride semiconductor layer. When an n-type layer, a p-type layer of the nitride semiconductor layer are formed on the different material substrate successively, eliminating the different material substrate (sapphire, etc.) after attaching the supporting substrate exposes the surface of the n-type layer. A damaged layer is formed in the surface of the n-type layer by eliminating the different material substrate with polishing. However, the damaged layer can be eliminated by chemical polishing, therefore eliminating the different material substrate may not reduce its characteristics.
The conductive layer is formed by a eutectic junction in the attaching step. The attaching step is performed by thermocompression bonding. It is preferable that the temperature is 150-350xc2x0 C. In the case more than or equal to 150xc2x0 C., it can accelerate diffusion of the metal of the conductive layer, so that the eutectic with uniform density distribution can be formed. Thus, it can improve intimate contact between the nitride semiconductor element and the supporting substrate. In the case over the 350xc2x0 C., the region of the diffusion may spread to the attaching region, so that it may reduce the intimate contact. The eliminating step eliminates the different material substrate by laser irradiation, polishing, or chemical polishing. The above step can make the exposed surface of the nitride semiconductor element mirror-like surface.
The method further includes an asperity-portion-forming step for forming an asperity portion on the exposed surface of the nitride semiconductor, which is the second type conductive nitride semiconductor layer, after the different-material-substrate-eliminating step. It can make the emitted light to be diffused at the asperity portion. Therefore, the light, which had total internal reflection conventionally, can be directed upward, and can outgo to outside of the element.
The method further includes a step for forming a second insulating protect layer on the exposed surface of the nitride semiconductor, which is the second type conductive nitride semiconductor layer, after the different-material-substrate-eliminating step. It can prevent short circuit when chipping by dicing, etc. to separate into chips. SiO2, TiO2, Al2O3, and ZrO2 can be employed as the protect layer. The method further includes a step for forming an asperity portion on the second insulating protect layer. It is preferable that the refractive index of the second insulating protect layer is more than or equal to 1 and not more than 2.5. Because the refractive index of the second insulating protect layer is between the nitride semiconductor element and the air, the outgoing efficiency of the light can be improved. It is more preferable that it is more than or equal to 1.4 and not more than 2.3. The constitution mentioned above can achieve more than or equal to 1.1 times of the outgoing efficiency of the light as much as that without the protect layer. The protect layer also can prevent surface deterioration.
The method further includes a step for breaking the nitride semiconductor into chips by etching the exposed surface of the nitride semiconductor after the different-material-substrate-eliminating step. In the light-emitting element of the invention, first, the semiconductor 2 is etched from the light-outgoing side until the first insulating layer 4, then the light-emitting element is formed into chips on the supporting substrate 11, to form the extending region of the first protect layer 4. At that time, though the semiconductor 2 is separated individually, the supporting substrate is not separated, in the wafer. Subsequently, the second insulating protect layer 7 is formed on the semiconductor 2 and the extending region of the first protect layer 4 except wire-bonding region of the second terminal 6. Forming the second insulating protect layer 7 on the side surfaces and the top surface of the semiconductor 2 can reduce physical damages cause of electric shorting and dust attachment. Next, after the second insulating protect layer 7 is formed, the light-emitting element is chipped by dicing from the supporting substrate 11 side. Consequently, a chip of the light-emitting element is obtained.
Subsequently, the light-emitting device is formed. First, the light-emitting element is mounted on a heat sink with lead frames, then conductive wires are bonded from the light-emitting element to the lead frames. After that, transparent glass packages it, and the light-emitting device is obtained (FIG. 19).
In a light-emitting device as another example, a package resin with a heat sink is prepared, and the light-emitting element is formed on the heat sink, then conductive wires are bonded from the light-emitting element to the lead frames. Subsequently, mold resin such as silicone is applied on the light-emitting element. Further, a lens is formed thereon, and the light-emitting device is obtained (FIG. 20).
It is preferable that the light-emitting device has a protect element for static protection of the light-emitting element.
The invention can improve the outgoing efficiency of the light extremely without increasing its voltage. The invention provides the opposed terminal structure, so that selecting the supporting substrate can improve thermal dissipation and life characteristics. Employing the conductive substrate as the supporting substrate can provide a one-wire structure. In addition, the conductive supporting substrate is employed, so that die-bonding to a package such as a lead frame by a conductive material can provide continuity. Therefore, it is not necessary to provide a pad terminal for a first terminal, so that the area of light-emission can be increased. When the face-down structure (n-side is surface) is used, the outgoing efficiency of the light can be improved. Additionally, the opposed terminal structure can widen the diameter. Providing asperity and aluminum at boundary surface thereof reflects the light, so that it can improve the outgoing efficiency of the light.
The method for producing a nitride semiconductor element of the invention can provide the nitride semiconductor element with the nitride semiconductor layer having fewer nicks or cracks occurred at exfoliation and with high thermal dissipation.
Further, the nitride semiconductor element of the invention has the coating layer including the fluorescent material, which can absorb a part of or the whole of the light from the active layer then can emit light with different wavelength, to emit the light with various wavelengths. Especially, it is preferable for a light source of illumination to include YAG so as to emit white light.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.